Obtaining an indication about a fat content of milk

A breast pump device (1) is equipped with or used in conjunction with an acoustic milk expression assessment system (6) for the purpose of obtaining an indication about a fat content of expressed milk. The acoustic milk expression assessment system (6) includes an acoustic sensor (61) and a processor (62) configured to process an acoustic signal received from the acoustic sensor (61) during operation of the breast pump device (1) when a milk receptacle (4) is used with the device (1). By recording sound during a pumping session, it is possible to determine a frequency shift in the sound of droplets falling down in the receptacle (4) and hitting a surface of the milk contained by the receptacle (4) with respect to a reference situation of a liquid having 0% fat content, which can be taken as a factor in estimating a value related to the fat content of the milk.